The present invention relates to a method and a device for switching from the freewheeling operating state of an electric machine to the short-circuit operating state.
Furthermore, the invention relates to a power converter and also an electrically driven motor vehicle comprising a device for switching from the freewheeling operating state of an electric machine to the short-circuit operating state.
Electric machines, such as permanently excited synchronous machines, for example, are used in numerous technical fields. By way of example, such permanently excited synchronous machines are used in motor vehicles. For safety reasons it is necessary here to take precautions for a safe operating state in the case of a fault. One possibility for a safe operating state is the so-called active short circuit, for example. In this case, the phase terminals of the electric machine are short-circuited simultaneously by means of suitable switching elements. A further safe operating state is so-called freewheeling. In this case, all the terminals of the electric machine are isolated from one another and no active driving of the electric machine with a voltage takes place.
The German patent application DE 10 2011 081 173 A1 discloses a device and a method for setting operating states of an inverter which drives an electric machine. In this case, it is possible to switch to freewheeling or an active short circuit by means of the inverter depending on the rotational speed of the electric machine.
During the freewheeling operating state of an electric machine, the induced voltage rises continuously as the rotational speed increases. The closing of the power switches of the inverter in order to change from the freewheeling operating state to the short-circuit operating state is accompanied by the occurrence of high overvoltages and/or overcurrents between the inverter and the electric machine in the individual phases. These can lead to damage or destruction of the components of the inverter or of the components connected to the inputs and outputs.
Therefore, there is a need for a method and a device in which the switching, the transition or the change of the operating state from freewheeling to the active short circuit is implemented in such a way as to minimize the overvoltages and/or overcurrents at and in the inverter and in the electric machine.